Thin LED flash for camera

ABSTRACT

A thin flash module for a camera uses a flexible circuit as a support surface. A blue GaN-based flip chip LED die is mounted on the flex circuit. The LED die has a thick transparent substrate forming a “top” exit window so at least 40% of the light emitted from the die is side light. A phosphor layer conformally coats the die and a top surface of the flex circuit. A stamped reflector having a knife edge rectangular opening surrounds the die. Curved surfaces extending from the opening reflect the light from the side surfaces to form a generally rectangular beam. A generally rectangular lens is affixed to the top of the reflector. The lens has a generally rectangular convex surface extending toward the die, wherein a beam of light emitted from the lens has a generally rectangular shape corresponding to an aspect ratio of the camera&#39;s field of view.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a 371 application of InternationalApplication No. PCT/IB2014/067204 filed on Dec. 22, 2014 and entitled“Thin LED Flash for Camera.”which claims benefit of U.S. Provisionalpatent application Ser. No. 61/923,925 filed on Jan. 6, 2014.

FIELD OF THE INVENTION

This invention relates to packaged phosphor-converted light emittingdiodes (pcLEDs) and, in particular, to a packaged pcLED that is usefulas a flash for a camera.

BACKGROUND

In modern digital cameras, including smartphone cameras, it is common toprovide a flash that uses a pcLED. A common flash is a GaN-based blueLED die mounted in a round reflective cup on a rigid printed circuitboard. A layer of YAG phosphor (emits yellow-green) fills the cup. Sincethe LED die is very thin, almost all light is emitted from the topsurface of the die. A circular Fresnel lens is then positioned over thecup to create a generally conical light emission pattern (having acircular cross-section) to illuminate a subject for the photograph. Acover plate, forming part of the camera body, typically has a circularopening for the lens.

Since the field of view of the camera is rectangular, much of the lightemitted from the flash, having a circular cross-section, illuminatesareas surrounding the subject and is wasted. Such unnecessaryillumination may also be bothersome to those not in the picture.

Further, due to the shape of the cup and the phosphor in the cup, thephosphor is not uniform over the LED die, resulting in colornon-uniformity vs. angle.

Further, due to the use of the rigid printed circuit board, the thinnessof the flash module is limited.

Further, the lens must be spaced away from the top surface of the LEDdie by a certain minimum distance (e.g., the focal length) in order toproperly redirect the light. This minimum distance significantly adds tothe thickness of the flash module.

Further, there is substantial back-reflection from the lens back towardthe cup and LED die.

Further, the bottom inner edge of the reflector cup facing the sides ofthe LED die has a thickness that is typically greater than the height ofthe LED semiconductor layers, so the inner edge of the cup blocks theside light or reflects it back into the LED die.

Further, since almost all light is emitted from the top surface of theLED die, the reflective cup has limited usefulness in shaping the beam,and the resulting beam is not very uniform across the field of view ofthe camera.

Further, since almost all light is emitted from the top surface of theLED die in a Lambertian pattern, the reflector cup has to have relativehigh walls to redirect and collimate the “angled” light emitted from theLED die. Any light rays that are not reflected (collimated) spread outat wide angles. The high walls of the reflector limit the minimumthickness of the flash module.

It is known to affix a lens over the LED die for a flash, where the lenshas a cavity for the LED die, such as described in patent publicationKR2012079665A. The lens has a rectangular top surface and curved sidesurfaces. However, a significant portion of the light escapes from thesides and is not reflected toward the subject. Also, the prior art lensis relatively thick, resulting in a thick flash module.

What is needed is a thin LED flash module for a camera that moreuniformly and efficiently illuminates a subject.

SUMMARY

In one example of the inventive flash module for a camera, a blueflip-chip LED die has a relatively thick transparent substrate on itstop surface. This causes a significant portion of the light emission tobe from the sides of the LED die, such as 50% of the total lightemission.

A conformal coating of phosphor is deposited over the top and sidesurfaces of the LED die to create a uniform white light.

The pcLED is mounted on a supporting substrate having a metal patternfor connection to the bottom anode and cathode electrodes of the LEDdie. The substrate has bottom metal pads for bonding to a camera'sprinted circuit board. The substrate can be a very thin flex circuit ora rigid substrate such as ceramic.

A rectangular reflector, with rounded corners, is then mounted on thesubstrate surrounding the rectangular LED die. The rectangular reflectorhas curved walls for redirecting the side LED light into a generallypyramidal beam, having a rectangular cross-section, where thecross-section aspect ratio is similar to the standard aspect ratio of acamera's field of view. In one embodiment, the reflector is stampedaluminum, where the opening for the LED die has knife edges facing thesides of the LED die so virtually all side light is reflected upwardrather than being blocked by the inner edges of the opening. Such aknife edge could not be achieved by a molded reflector cup.

A thin lens is affixed over the top of the reflector, where the lens hasa convex side that faces toward the LED die, so the convex portion doesnot add thickness to the module. The lens not only protects the LED diebut increases light extraction due to the convex portion receiving mostof the light from the LED die and reflective walls at a substantiallynormal angle. In contrast to a prior art conventional Fresnel lens,having a flat surface facing the LED die, there is much lessback-reflection.

Due to the high percentage of side light being reflected upward by thereflector, the effective optical distance between the lens and the LEDdie is the sum of the horizontal distance between a side of the LED dieand a curved wall of the reflector plus the vertical distance betweenthe curved wall and the lens. Therefore, the lens can be spaced a focallength from the LED die's sides while being much closer to the topsurface of the LED die. This allows the flash module to be even thinner.The reflector walls can be more widely spaced from the LED die tofurther reduce the thinness of the module.

Since a large portion of the light emitted from the LED die is from itssides, the reflector walls can be made relatively shallow, furtherreducing the thinness of the module.

The convex shape of the lens toward the LED die and the closeness of thelens to the LED top surface cause the lens to intercept a wide angle ofthe Lambertian light emitted from the top surface of the LED die andslightly redirect it toward the center of the beam, if necessary, tofurther improve the uniformity of the beam. The convex shape is designedto optimize the uniformity of light across a desired portion of thegenerally rectangular beam. The lens is not used to significantly shapethe beam (but primarily improves uniformity), since the shape of thebeam is primarily controlled by the shape of the reflector, in contrastto the prior art.

Accordingly, due to the large percentage of the LED die light being sidelight and redirected by the rectangular reflector, and the reflectedlight being blended with the light emitted from the top surface of theLED die, a more uniform rectangular beam of light is emitted by theflash, which generally matches the aspect ratio (e.g., 4:3) of thecamera's field of view. Further, the flash module can be made very thin.

Other embodiments are described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of a flash module in accordance with oneembodiment of the invention.

FIG. 2 is a cross-sectional compressed view of an LED die, with aconformal phosphor coating, mounted on a flex circuit (or othersupporting substrate) that may be used in the module of FIG. 1.

FIG. 3 is a cross-sectional view of the flash module of FIG. 1.

FIG. 4 is a top down view of the flash module of FIG. 1.

FIG. 5 is a bottom view of the flash module of FIG. 1 showing theelectrode pattern and thermal pad.

FIG. 6 is a perspective bisected view of the flash module of FIG. 1.

FIG. 7 is a perspective view of the flash module of FIG. 1.

FIG. 8 is a cross-sectional view of an embodiment of the flash moduleusing a flex substrate and identifying various dimensions inmillimeters.

FIG. 9 is a back view of a smartphone illustrating the rectangular flashmodule and camera lens.

Elements that are the same or similar are labeled with the same numeral.

DETAILED DESCRIPTION

FIG. 1 is an exploded view of a flash module 10 in accordance with oneembodiment of the invention. A support substrate 12 may be a rigidsubstrate or a very thin flexible circuit. Using a flexible circuit asthe support substrate 12 allows the module 10 to be thinner.

A metal trace 14 pattern is formed on the substrate 12 to define metalpads 15 and 17 for the bottom anode and cathode electrodes of the flipchip LED die 16 and to define metal pads 19 and 21 for the electrodes ofan optional transient voltage suppressor (TVS) chip 18.

The bare LED die 16, such as a GaN-based blue LED die, is thenelectrically and thermally connected to the substrate 12. The TVS chip18 may also be electrically connected to the substrate 12.

Typically, the LED die 16 is a flip chip die, although other die types,including those with bonding wires, may be used. To minimize thethickness of the flash module 10, the LED die's bottom electrodes aredirectly bonded to the metal pads 15 and 17 of the metal trace 14. Inanother embodiment, the bare LED die 16 may be first mounted on asubmount with more robust bottom metal pads to simplify handling andenable the LED die 16 to be conformally coated with a phosphor layer 20after bonding to the submount. FIG. 1 shows an alternative embodimentwhere the phosphor layer 20 covers only the top surface of the LED die16.

If the bare LED die 16 is directly mounted on the substrate 12 (as shownin FIG. 2), the phosphor layer 20 may be deposited over the entiresubstrate 12 and LED die 16 to coat the top and side surfaces of the LEDdie 16. The phosphor layer 20 may be phosphor particles, such as YAGphosphor particles or red and green phosphor particles, infused in asilicone binder. The phosphor layer 20 may also serve as an adhesivelayer for affixing a reflector 22 to the surface of the substrate 12. Inthe alternative, a separate adhesive may be used to affix the reflector22.

Various details of the components in FIG. 1 are described with respectto FIGS. 2-8.

FIG. 2 is a cross-sectional compressed view of the LED die 16 with theconformal phosphor layer 20. Although a flip-chip die is shown in theexamples, the present invention is applicable to any type of LED die,including vertical LED dies, lateral LED dies, etc.

The LED die 16 includes a bottom anode electrode 23 bonded to the metalpad 15 (defined as a portion of the metal trace 14 of FIG. 1) andincludes a bottom cathode electrode 25 bonded to the metal pad 17. Thepads 15 and 17 are electrically connected by vias 30 and 31 toassociated bottom pads 32 and 34, which may be used to solder the flashmodule 10 to a camera's printed circuit board. A thermal pad 36 isformed on the bottom surface of the substrate 12, which may be solderedto a heat sink in the printed circuit board.

The LED die 16 semiconductor layers are grown on a relatively thicksapphire substrate 40, which may be as thick as 1 mm. This is thickerthan a typical growth substrate, since a manufacturer typically uses thethinnest growth substrate practical for reducing costs and maximizingthe top emission. Frequently, in the prior art, the growth substrate iscompletely removed. The sapphire substrate 40 is much thicker thanrequired for mechanically supporting the LED semiconductor layers. Othermaterial for a growth substrate may instead be used. The top surface andgrowth surface of the growth substrate 40 may be roughened forincreasing light extraction.

A typical width of the LED die 16 is on the order of 1 mm.

N-type layers 42 are epitaxially grown over the sapphire substrate 40,followed by an active layer 44, and p-type layers 46. A portions of theactive layer 44 and p-type layers 46 are etched away to gain electricalcontact to the n-type layers 42 by means of a via 48 leading to thecathode electrode.

The active layer 44 generates light having a peak wavelength. In theexample, the peak wavelength is a blue wavelength, and the layers 42,44, and 46 are GaN-based.

Alternatively, the growth substrate 40 may be removed and replaced by atransparent support substrate, such as glass, affixed to thesemiconductor layers by an adhesive (e.g., silicone) or by othertechniques.

By using a thick growth substrate 40 (or other transparent substrate),the light exiting the sides of the LED die 16 is made to be preferablyabout 50% of the total light emission, with 50% of the total light beingemitted from the top surface of the LED die 16. In another embodiment,over 30% of all light emitted by the LED die 16 is from the sides, wherethe percentage of side light is based on the thickness of the substrate40. The more side light, the more the reflected light from the reflector22 is adding to the overall beam and the thinner the flash module canbe.

In one embodiment, the thickness of the LED semiconductor layers is lessthan 100 microns (0.1 mm), and typically less than 20 microns, and thesubstrate 40 thickness is greater than 0.2 mm and up to 1 mm.

A portion of the blue light leaks through the phosphor layer 20, and thecombination of the blue light and phosphor light creates white light forthe flash. Since the phosphor layer 20 has a uniform thickness, thecolor emission will be substantially uniform vs. angle.

The reflector 22 (FIG. 1) is preferable formed by stamping an aluminumsheet. The stamp forms a rectangular opening 52 in the sheet andcompresses the surrounding aluminum to form curved sidewalls 54. Theterm “rectangular,” as used herein, includes a square, and includesrectangles with rounded corners. The edges of the opening 52 are knifeedges (less than 50 microns thick) to limit any back reflection of thelight emitted from the sides of the LED die 16/phosphor. Typically, theopening 52 and curved sidewalls 54 have the same aspect ratio as thecamera's field of view, such as 4:3, so the resulting beam will resemblethe 4:3 aspect ratio.

The reflector 22 is then coated with a silver layer for highreflectivity, such as by plating, evaporation, sputtering, etc.

The footprint of the reflector 22 may be approximately that of thesubstrate 12 to minimize the size of the flash module 10. The reflector22 is then affixed to the substrate 12 using the phosphor layer 20(containing silicone) as an adhesive. The reflector 22 adds rigidity tothe module 10. The phosphor layer 20 is then cured.

A preformed polycarbonate lens 56 is then affixed to the top surface ofthe reflector 22, such as by silicone. The silicone is then cured tocomplete the flash module 10. Typically, the lens 56 is rectangular withrounded edges to receive the generally rectangular emission from thereflector 22 and LED die 16.

As shown by the cross-sectional view of the module 10 in FIG. 3, thelens 56 has a flat top surface and a bottom surface. A portion of thebottom surface is a convex surface 58 that faces the LED die 16. Thus,the convex surface does not add to the thickness of the module 10.Typically, the convex surface 58 is rectangular or rectangular withrounded corners, as shown in FIG. 1.

In prior art flash modules using LED dies that generate little sidelight, the lens had to be spaced relatively far from the top surface ofthe LED die to properly redirect the light. In one embodiment of presentinvention, around 40-50% of the light is emitted from the sides of theLED die 16, and the effective optical distance from the LED die 16 tothe lens 56 is the sum of the generally horizontal distance from an LEDdie side to a reflector wall 54 plus the generally vertical distancefrom the reflector wall 54 to the lens 58. Accordingly, to make themodule 10 even thinner, the reflector walls 54 can be further spacedfrom the LED die 16 while retaining the same effective optical distancebetween the sides and the lens 58. The lens 56 is designed to improvethe uniformity of light across a central portion of the rectangularbeam.

In one embodiment, the effective optical distance between the sides ofthe LED die 16 and the lens 58 is approximately the focal length of thelens 58.

Dry air (or other gas) fills the gap between the lens 56 and the LED die16 to obtain a large difference in the indices of refraction at theinterface of the lens 56 and the gap to achieve the desired refractionby the lens 56.

FIG. 3 shows two sample light rays 60A and 60B. Rays, such as 60A, fromthe top center surface of the LED die 16 are not substantiallyredirected by the lens 56. Reflected rays, such as ray 60B, that impingethe convex surface 58 at an angle are slightly redirected toward thecenter axis to improve the uniformity of the beam across at least acentral portion of the 4:3 aspect ratio. The shape of the beam isprimarily defined by the shape of the reflector 22, since the reflector22 reflects virtually all side light and some angled light from the topsurface of the LED die 16.

FIG. 3 also shows that the aluminum sheet for forming the reflector 22is stamped to have a bottom cavity for the TVS chip 18.

By using the phosphor layer 20 (a dielectric) as an adhesive for thealuminum reflector 22, the bottom of the metal reflector 22 does notshort out the metal traces 14, and there is no separate step fordepositing an adhesive. In another embodiment, the reflector 22 isformed to have a thin dielectric layer on its bottom surface beforebeing mounted on the substrate 12.

FIG. 4 is a top down view of the module 10 of FIG. 3.

FIG. 5 is a bottom view of the module 10 showing the cathode and anodebottom pads 32 and 34, and the thermal pad 36, also shown in FIG. 2.

FIG. 6 is a perspective bisected view of the flash module 10. Thephosphor layer 20 over the sides of the LED die 16 is not shown.

FIG. 7 is a perspective view of the flash module 10 of FIG. 1.

FIG. 8 is a cross-sectional view of an embodiment of the flash module 10using a flex substrate 12 and identifying various dimensions inmillimeters. Although the LED die 16 is about 1.0 mm in width, theheight of the lens 56 above the top surface of the LED die 16 is onlyabout 0.3 mm, since the optimal separation is based on the travel pathof the side light to the lens 56 when being reflected off the reflector22.

The flex substrate 12 only adds 0.05 mm to the thickness of the module10. The phosphor layer 20 is shown as being 0.05 mm thick. The reflector22 is shown as being 0.750 mm thick, and the lens 56 is shown as addingonly 0.1 mm to the module 10. The growth substrate 40 (FIG. 2) may beabout 0.25-0.5 mm thick. The total height of the flash module 10 of FIG.8 is less than 1 mm. It is envisioned that all practical flash modulesof the invention, using a flex circuit, can be formed to havethicknesses less than 2 mm.

Note that the top surface area of the LED die 16 is about 1 mm² and thecombined area of the four sides of the LED die, using a 0.5 mm thicksubstrate 40, is about 2 mm². For a substrate 40 thickness of 0.25 mm,the side area equals the top surface area. So there is substantial sideemission.

FIG. 9 is a back view of a smartphone 66, illustrating the rectangularflash module 10 and camera lens 68.

Accordingly, the present invention reduces the thickness of a flashmodule, improves the color uniformity across the beam, and increases theefficiency of the flash by creating a generally rectangular beam with asubstantially uniform intensity across the relevant portion of the beamand by incurring less reflection of the LED light back toward the LEDdie.

The present invention may be used for other applications besides cameraflashes, such as a flashlight module.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications asfall within the true spirit and scope of this invention.

What is claimed is:
 1. A light emitting device comprising: a support structure; a light emitting diode (LED) die mounted on the support structure, the LED die having a top surface and at least one side surface and comprising LED semiconductor layers and a transparent substrate, the substrate comprising the top surface of the LED die so that the LED die has anode and cathode electrodes on a bottom surface, the substrate being thicker than the LED semiconductor layers, wherein light emitted from the side surfaces is at least 30% of all light emitted by the LED; a phosphor layer covering the top surface and side surfaces; a reflector surrounding the LED die, the reflector having curved surfaces surrounding the LED die, wherein the curved surfaces extend from a generally rectangular opening for the LED die and reflect the light from the side surfaces of the LED die to form a generally rectangular beam; and a generally rectangular lens affixed to walls of the reflector, the lens having a generally rectangular convex surface extending toward the LED die and a flat top surface facing away from the LED die.
 2. The device of claim 1, wherein the device is a flash for a camera.
 3. The device of claim 2, wherein an aspect ratio of the beam is substantially the same as an aspect ratio of a field of view of the camera.
 4. The device of claim 1, wherein the support structure is a flexible circuit.
 5. The device of claim 1, wherein the reflector comprises a stamped metal reflector, where the opening has a knife edge facing the LED die.
 6. The device of claim 1, wherein the lens has a focal length and wherein a distance from a side of the LED die, taken normal to the side of the LED die, to the reflector plus a distance from the reflector to the lens is approximately the focal length.
 7. The device of claim 1, wherein light from the at least one side surface of the LED die is at least 40% of all light emitted by the LED die.
 8. The device of claim 1, wherein light from the at least one side surface of the LED die is at least 50% of all light emitted by the LED die.
 9. The device of claim 1, wherein the substrate is a growth substrate for the LED semiconductor layers.
 10. The device of claim 1, wherein the phosphor layer conformally coats the top surface and the at least one side surface and has substantially uniform thickness.
 11. The device of claim 1, wherein the phosphor layer comprises phosphor particles infused in a binder, wherein the phosphor layer covers a portion of a top surface of the support structure, and wherein a bottom surface of the reflector is affixed to the support structure by the binder acting as an adhesive.
 12. The device of claim 1, wherein a footprint of the reflector is substantially the same as a footprint of the support structure.
 13. The device of claim 1, wherein a total height of the device is less than 1 mm.
 14. The device of claim 1, wherein a total height of the device is less than 2 mm. 